In lubricant systems it is typical to provide a cyclically operable lubricant pumping apparatus which is periodically operated to build lubricant pressure to a suitable level. The lubricant under pressure is then metered to provide a measured flow of the lubricant to the equipment being so lubricated.
Since the metering of the flow of lubricant is adjusted to provide lubricating fluid sufficient for a substantial operating interval, the pump is typically operated to supply lubricant at intervals separated by substantially long time periods which are usually of the order of hours. However, it is very important to be assured that the pump is in fact operating properly since failure in providing adequate lubricant flow may cause severe damage to the equipment being so lubricated, necessitating the use of circuitry which is capable of developing time delays which are of the order of a few hours.
Various systems have been devised to ascertain whether automatically operating cyclic liquid dispensing apparatus is operating properly or is malfunctioning. A conventional sensing device for determining whether there has been a malfunction comprises a pressure sensing device for sensing system pressure after each periodic pulse of liquid pressure. Such devices are coupled with a timing device for generating a time interval that is slightly longer than the time that elapses between pump operations, so that if a pulse of liquid at the correct pressure is not sensed within a predetermined timer period, an indication is given that a malfunction has occurred. The pressure sensing device is associated with an automatic reset timer. The pressure sensing device resets the timing device by activating a reset device in the timing device. An indicator device is connected with the pressure sensing device to indicate when the pressure sensing device has responded to an underpressure due to a malfunction in the system. Such prior art systems are expensive since they require a pressure sensing device, a complex and expensive timer and a complex and expensive recycling means.
An effort to obtain similar operations through a less expensive apparatus led to the development of the hydraulic pressure cycle indicator described in U.S. Pat. No. 3,583,528. One disadvantage of this system resides in the fact that large time variations occur due to changes in lubricant viscosity with ambient temperature thereby significantly altering the bleed-off interval.